1. Field of the Invention
The present invention relates to an imaging lens which forms an image of a subject on an image pickup device such as a CCD (Charge Coupled Device), a CMOS (Complementary Meal Oxide Semiconductor) and the like. The invention also relates to an imaging apparatus for converting an image of a subject which is formed on the image pickup device into an electric signal so as to obtain a digital image of the subject which is used as a mobile phone with a camera, a portable information terminal (PDA: Personal Digital Assistance), a digital still camera, an on-board camera and a monitoring camera, and an image reader for reading an image such as on a film.
2. Description of the Related Art
In recent years, there has been a demand for smaller lenses in the field of camera lenses which are used in mobile phones with a camera, portable information terminals and the like because of smaller installation spaces, and lenses that are bright and which have high resolving powers have also been in demand because of an upward trend of larger image pickup devices and highly fined pixels. In addition, the same issues are occurring with imaging lenses for digital still cameras and reading lenses for film scanners. To meet these demands, there have been proposed a number of small, bright and highly resolving lenses which are corrected with respect to several aberrations including chromatic aberration. Conventionally, for example, in many cases, imaging lenses which were installed in mobile phones with a camera were made up of three lens elements. However, in order to meet the increase in the number of pixels, there is a tendency that the number of lens elements is increased. JP-A-2004-184987, JP-A-2004-212481, JP-A-2005-4027 and JP-A-2005-31638 describe imaging lenses which are made up of four lens elements.
While image pickup devices are enlarged in size as the resolving power or number of pixels is increased, with any of the imaging lenses described in the aforesaid patent documents which are made up of four lens elements, correction of curvature of image and chromatic difference of magnification is insufficient for installation in an image pickup device which is ½ inch or larger in size, and hence, the imaging lenses described in the patent documents are not suitable for an image pickup device which requires a high resolving power. In addition, in the patent documents, plastic lenses are used in some examples thereof, and polycarbonate is used as a plastic material for the plastic lenses in some examples. With this material used for plastic lenses, since a double refraction is easy to occur at the time of molding, optical path lengths of light rays which pass through a lens made from polycarbonate are desirably made as short as possible, and hence, when a polycarbonate plastic lens is used for a lens which requires a high resolving power, for example, a thickness of the lens at the center thereof needs to be restricted, this decreasing the degree of freedom in design. Consequently, such poly carbonate plastic lenses are said to be unsuitable for use as a lens requiring a high resolving power.